1. Technical Field of the Invention
The present invention relates generally to a color temperature automatic adjusting system for a color television receiver. More specifically, the invention relates to an automatic white balance adjusting system which can eliminate unstable pictures in the transition period after turning on the power switch of the color television receiver.
2. Background Art
Television receivers contain automatic white balance circuits of the type disclosed in published European patent application No. 0 192 413 (corresponding to U.S. application Ser. No. 826,025, filed Feb. 4, 1986). Such circuits monitor the cathode beam currents for the cathode ray tube and adjust the gains of the primary color signal circuits accordingly to maintain a white balance.
A problem arises, however, when the television receivers are first switched on. Since the heater for the cathode electrode of the display tube is cold when the television receiver is turned on, the primary color signal circuits are controlled so as to be in a condition to cause the cathode beam currents to flow at a maximum value. Thus, when the heater is heated sufficiently, and the cathode emission begins, the video screen becomes too bright during a transient time until the auto white balance circuit becomes stabilized. Also, since the cut-off voltage EKCO of the display tube is scattered between the red color, the green color and the blue color cathodes, the auto white balance circuit will be stabilized after different times for red, green and blue, and the color of the picture will fluctuate.
The Japanese Patent First Publication (Tokkai) No. Showa 55-67286 discloses a white balance adjusting apparatus which attempts to overcome this problem. In the disclosed system, the automatic control loop is cutoff and a white balance adjusting voltage is supplied from a predetermined bias source upon turning on the power supply for a television receiver. The cutoff condition of the automatic control loop is maintained for a predetermined period of time after turning on the power supply. Upon expiration of the predetermined period, the power supply for the automatic adjusting loop is resumed. Therefore, the automatic adjusting loop becomes operative after a given delay time corresponding to the aforementioned predetermined period after turning on the power supply. Then, automatic white balance adjustment takes place by means of the automatic control loop.
However, since the picture is immediately displayed in response to turning on the power supply for the television receiver, an unadjusted picture becomes visible while the automatic adjusting loop is held inoperative. Furthermore, even after resumption of the operation of the automatic control loop, the picture condition becomes unstable during the transition period while the white balance adjustment takes place. Therefore, in the system disclosed in the Tokkai Showa 55-67286, it is inevitable that an unstable picture condition occurs during the turn on transition period.